A digital versatile disk (referred to as DVD) that is a typical optical disk in recent years is in widespread use in very wide range of fields, from readout systems such as movies, music, games, and car navigation to recording systems such as TV recorders and personal computer mounting drives. Recording capacity thereof is approximately seven times as much as CD, for example, it is sufficient capacity for recording the current TV video in standard image quality. However, when digital hi-vision video to be expected to become widespread in future is directly recorded in high quality, amount of information overwhelmingly increases as compared before; and therefore, the current DVD can only record approximately 20 to 30 minutes. Consequently, a next generation optical disk system which is capable of recording large capacity as much nearly 5 times as conventional DVDs is becoming increasingly expected.
As a light source for use in a next generation optical disk system, a nitride based blue-violet semiconductor laser with a wavelength of 405 nm is used. This is because light can be narrowed down as compared with an AlGaInP based red semiconductor laser with a wavelength of 650 nm used in conventional DVDs and is suitable for high density recording of signals.
An existing nitride based semiconductor laser is not easy for manufacturing due to a material inherent property and it is the common practice to have a ridge structure by dry etching a p-type cladding layer. More specifically, as shown in FIG. 4, an n-type substrate 11, an n-type cladding layer 13, an active layer 15, and a p-type cladding layer 20 which is of a ridge structure are accumulated in this order. Further, the lower surface of the n-type substrate 11 and the upper surface of a ridge portion of the p-type cladding layer 20 are provided with electrodes 22 and 23, respectively. In the case of the nitride based semiconductor laser having the ridge structure, horizontal optical confinement is controlled by refractive-index difference with outside air of the ridge structure. Therefore, the post-etching remaining thickness d of the p-type cladding layer 20 is important parameters for deciding beam shapes.
However, there arise problems as follows. First, since outside the ridge is air with small refractive indexes, a horizontal refractive-index difference Δn comparatively increases. Therefore, it tends to generate a higher transverse mode and it is difficult to perform single transverse mode operation with a high output. The remaining thickness d of the p-type cladding layer 20 needs to be increased in order to reduce Δn in the nitride based semiconductor laser with the ridge structure. In this case, since horizontal current broadening in the p-type cladding layer 20 increases; ineffective current components which do not contribute to laser oscillation are increased to cause increase of operation current.
Furthermore, since outside the ridge structure is air which is low in thermal conductivity, most heat radiation from an emitting region is performed only from the n side. Therefore, the nitride based semiconductor laser having the ridge structure is low in radiation performance and difficult in high output and operation in high temperature.
Further, a ridge waveguide is formed by processing the p-type cladding layer 20 with a method such as dry etching. Therefore, it tends to generate variation in the remaining thickness d of the p-type cladding layer 20 which is extremely important for control of a beam shape; and this is one of factors which make yield low.
In order to solve these problems, there is proposed a nitride based semiconductor laser having an inner stripe structure, disclosed in Patent Document 1, for example. As shown in FIG. 5, in the case of this structure, since outside the stripe is covered with a nitride based material, horizontal refractive-index difference Δn is relatively low. Therefore, single transverse mode operation with a high output can be easily realized.
Furthermore, outside the stripe is covered with a nitride based material having high coefficient of thermal conductivity, and therefore, radiation performance is high and high output and operation in high temperature can be made.
Further, since the portion d corresponding to the remaining thickness in the p-type cladding layer of the ridge structure is formed in crystal growth, variation in layer thickness is small and a beam shape can be stably controlled. Therefore, it is a high yield and superior in mass productivity.
Additionally, since an area in contact with a p-electrode can be widely taken, contact resistance can be reduced as compared with the ridge structure as shown in FIG. 4.
[Patent Document 1] Japanese Laid-open Patent Publication No. 2003-78215